libxslt: fix impurity on 'native-stdenv' platforms

libxslt has optional dependencies which may be found in /usr or
/usr/local on platforms that have a native stdenv. With those features
enabled, the build generated binaries that depend on libraries outside
of the store. In this particular case, the NixOS channel had binaries
for FreeBSD that depended on libgcrypt, apparently because that packages
happens to be installed outside of Nix on the build machine. On other
machines, however, those binaries failed with unresolvable references.